1. Technical Field
The invention relates to vehicle tires and particularly to a run flat pneumatic tire containing a thin annular band which acts as a structural compression member when the tire is in the unpressurized or uninflated state to enable the tire to run in this condition. More particularly, the invention relates to a composite band element comprising a plurality of wound fibers contained in a resin matrix formed about a mandrel and which has an anticlastic shape with a concave surface facing radially outwardly to enhance inflated tire ride quality and band element durability for both inflated and uninflated conditions.
2. Background Information
Various tire constructions have been devised over the years which enable a tire to run in an under-inflated or non-inflated condition, such as after receiving a puncture and loss of pressurized air for extended periods of time and at relatively high speeds. This enables the operator to safely drive the vehicle to an appropriate location for repair or replacement of the punctured tire. Certain of these safety tires, referred to as xe2x80x9crun flat tiresxe2x80x9d, have been successful for certain applications and certain types of tire constructions. Most of these run flat tires achieve their run flat capability, by the placement of reinforcing layers or members of relatively stiff elastomeric material in the side walls of the tire which enable the tire to support the vehicle weight even with the complete loss of internal air pressure. Examples of such prior art run flat tire constructions which use such sidewall inserts are shown in U.S. Pat. Nos. 3,911,987; 3,949,798; 3,954,131; 4,067,372; 4,202,393; 4,203,481; 4,261,405; 4,265,288; 4,287,924; 4,365,659; 4,917,164; and 4,929,684.
In addition to these prior art run flat tires having side wall reinforcements, various run flat tire constructions have been developed which utilize a thin annular band which extends circumferentially throughout the tire beneath the tread area in the crown portion of the tire. Examples of such banded run flat tires are shown in U.S. Pat. Nos. 4,111,249; 4,318,434; 4,428,411; 4,459,167; 4,734,144; 4,428,411; 4,673,014; 4,794,966; and 4,456,048.
Banded tires have been fabricated with band materials made of steel, aluminum, titanium, and epoxy and thermoplastic composites with glass, KEVLAR (aromatic polyamide) and graphite fiber reinforcement. A possible failure mode with these lightweight, laminated band constructions is interlaminar shear along the band""s primary bending neutral axis. This is a fatigue failure and is directly related to the spectrum of cyclic operating stress. As in all fatigue failures, the lower the stress, the longer the operating life.
The inflated ride quality, comfort and harshness of a banded run flat tire is effectively controlled by the circumferential rigidity and bending stiffness of the band element within the central footprint region. The circumferential rigidity necessary for the band element to adequately carry tire loads in both the inflated and uninflated conditions, is largely determined by the bending stiffness of the band element ahead of and behind the central footprint region. It is desirable that the band element have variable axial bending stiffness in order to improve inflated ride quality, comfort and less hardness with the load carrying durability of the band element being improved by providing higher axial bending stiffness ahead of and behind the central footprint region. Likewise, it is desirable that the band element also has increased interlaminar shear strength and increased circumferential strength or stiffness in order to reduce failure of the band.
It has been found that band elements having dual stiffness characteristics have been effective in achieving a better run flat tire. A dual stiffness band has variable axial bending stiffness with a lower axial bending stiffness in the central footprint region which improves inflated ride quality, comfort and harshness, yet provides a higher axial bending stiffness ahead of and behind the central footprint region in order to provide the necessary band durability and load carrying capacity. Numerous band element configurations have been disclosed in the prior art which provide for a dual modulus result. One such band configuration for providing the dual modulus effect is to use a band element having an anticlastic shape such as shown in U.S. Pat. No. 4,456,084. An anticlastic band has an arcuate cross section with an outwardly concave configuration and under load will deform so as to have a lower section modulus where the band flattens out in the central footprint region and a more stable section modulus due to elastic deformation where the band radius of curvature is decreasing ahead of and behind the central footprint region. This movement of the instant flexural center is similar to what one sees in an extended curved section measuring tape, concave up vs. concave down. The prior art anticlastic band of U.S. Pat. No. 4,456,084 is formed of sheet metal material which was found to be extremely difficult to form into the anticlastic shape. It required complex fabrication in that the sheet material had to be rolled into the anticlastic shape and then the ends secured by some type of welding. Both of these fabrication procedures are difficult to perform in order to achieve a uniform band configuration. Likewise, the sheet metal material had to be hardened by heat treating, straining or other similar metallurgic techniques to achieve the desired strength.
In summary, the present invention provides a run flat tire and in particular a band element therefor, which has both increased interlaminar shear strength and which provides for a dual stiffness effect when the tire is operating both in the inflated and deflated conditions by providing an anticlastic band formed of a composite material.
Another feature is to provide a band element for a run flat pneumatic tire which is formed of a composite material from various types of fibers, preferably graphite, fiberglass or a combination of both, which fibers are encased in a resin matrix and provided with an anticlastic configuration to provide a dual stiffness band. The term xe2x80x9canticlasticxe2x80x9d is defined as a double curved shape where the surface curve is concave in one direction and convex in another. This dual stiffness provides an improved ride quality and comfort by having a lower axial bending stiffness in the central footprint region of the tire and provides the required durability and strength by providing a higher axial bending stiffness in the band ahead of and behind the central footprint region as the tire rotates.
Another aspect of the invention is to provide a band formed of wound layers of resin impregnated fiber tows which form a plurality of layers providing generally axial extending concave surfaces between the layers within the band, which surfaces cross the neutral axis of the band due to the curvature of the band, thereby increasing interlaminar shear strength.
Still another aspect of the invention is providing the anticlastic band in which the concavity thereof is within the range of {fraction (1/10)} to twice the band thickness, and in the preferred embodiment forming the concavity to equal the band thickness.
A further aspect of the invention is to enable the band to be fabricated by usual methods of fabrication available for prior composite band elements, such as homogenous filament winding, nonhomogenous filament winding, multi-layer tape composite winding, winding with preimpregnated materials, winding with wet woven materials, winding with mats, winding with resin transfer molding processes, winding with wet or preimpregnated woven preforms, and combinations of these prior known composite band forming techniques and methods of fabrication.
Another feature of the invention is to provide a band element formed of composite material which allows tailoring of both stresses and elastic deflection properties by controlling orientation of the fibers and selection of fiber modulii dependent upon the type of material chosen instead of the prior art sheet metal fabrication which is limited to the isotropic material properties of the metal.
Still another aspect of the invention is to provide an anticlastic band which is lighter in weight than prior sheet metal anticlastic bands, which is considerably easier to manufacture and is free of undesirable splices thereby eliminating possible undesirable irregularities in the band.
A further aspect of the invention is the ability to improve the processing and manufacturing of the anticlastic shape by use of various methods of forming the band on mandrels either collapsible or deformable, to enable the non-cylindrical shape of the band to be formed and then removed from a cylindrical mandrel.
Further advantages, construction and operation of the present invention will become more readily apparent from the following description and accompanying drawings.